Now my TH level is 7. Should I upgrade mortar from level 4 to 5 before upgrading TH to level 8?

Yes.
It is very important to upgrade your mortar to the highest possible level on a certain town hall level before you move to a higher town hall level.
An upgraded mortar has higher hit points and can cause more damage to attacking troops giving your village a stronger defense.
Always remember that a good defense is as important as aggressive attacking.
It is not just the mortar that you need to upgrade before moving to a higher town hall level but all your defensive buildings as well.
Even though how strong your offense is during raids and you get most of the loot, you cannot defend them on your village from other player’s raid if you have low level defensive buildings.